1. Field of the Invention
The present invention relates to a pre-filter for a servo control loop, and applications thereof.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., including part of, one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist).
During the lithography process, actuators are used to position various modules of the lithography apparatus. Often there is an error in the positioning, velocity or acceleration of modules. Errors in the position, velocity or acceleration of a module degrade the desired tool performance. If possible, the negative effect of these errors in performance is compensated by generating an error signal to control the position of one or more additional modules that correct the error or by actively modifying the position of existing modules. However, the error control signal frequently used for this purpose is itself prone to errors and may be reliable only for a certain frequency range